1. Field of the Invention
The present invention relates to a forward conversion switching power supply, and more particularly to a forward conversion switching power supply in a constant-voltage clamping form, which can prevent each of a magnetic saturation phenomenon in a main transformer, an excessive high voltage of a main switching transistor, and a shift effect of an input voltage, and can increase the duty cycle and improve an operating efficiency.
2. Description of Related Art
A switching power supply has been widely used as the power supply of a computer and in the uninterruptible power supply (UPS) system, since it not only has a small volume and weight, but also has an excellent power usage when it operates in high frequencies. To this end, various kinds of switching power supplies have been developed for appliances.
The most fundamental and typical kind of forward conversion switching power supply operates in a manner of a passive flyback magnetic type. Referring to FIG. 4, the kind of switching power supply comprises a primary rectifying filter circuit 93, a transformer 94, a secondary rectifying filter circuit 95, a sampling feedback circuit 91, a pulse width modulator 92 and a main transistor 921. A passive flyback magnetic circuit composed of a diode 941 and a resistor 943 and a capacitor 942 is connected to a primary winding of the transformer 94. The primary rectifying filter circuit 93 rectifies and filters the input alternating current (AC) to supply DC power. The pulse width modulator (PWM) 92 enables the main transistor 921 to switch on/off in high frequencies so that the transformer 94 can operate for purpose of a high-frequency magnetization. Then a secondary winding of the transformer outputs a high-frequency alternating voltage. The high-frequency alternating voltage is rectified and filtered by the secondary rectifying filter circuit 95 and then a desired DC power can be output. Also, the sampling feedback circuit (SFC) 91 sends out a reference voltage responsive to the output voltage OUT to the pulse width modulator 92 so that the pulse width modulator 92 can change the duty cycle with which the main transistor switches on/off according to the magnitude of the output voltage OUT. Thereby, the output voltage OUT can be stabilized at a desired value. Further, the passive flyback magnetic circuit (including the resistor 943, the capacitor 942, and the diode 941) can absorb the flyback magnetic current of the transformer 94. This kind of switching power supply has a disadvantage of great power loss. A second disadvantage is that the switching power supply can not reach a dynamic compensation when the load varies. In this case, a serious saturation current will occur in the transformer and abnormal waves may occur on the main transistor, as shown in FIG. 7B. Therefore, an element with high withstand voltage needs to be used as the main transistor. A third disadvantage is that the main transistor has an excessively high peak current (see FIG. 7), a poor response to the dynamic load and great noise.
Another conventional kind of switching power supply operates in the form of a a clamping coil, as shown in FIG. 5. The switching power supply has a basic circuit similar to that in FIG. 4. The difference is that the transformer 94 in FIG. 5 has a secondary winding 944 connected in series with a diode 945 and between the primary winding and the ground. The secondary winding 944 is provided for generating an induced current opposed to the primary winding to offset the magnetizing current. This kind of switching power supply also has a disadvantage that the main transistor must utilize an element with high withstand voltage, as shown in FIG. 7C. Another disadvantage is that a duty cycle thereof is adapted to be no more than 50% because the turns ratio of the secondary winding of the transformer 94 limits the effective duty cycle.
A third kind of switching power supply is a dynamic clamping type. Referring to FIG. 6, the switching power supply has a delay circuit 946, an additional transformer 97, and a clamping transistor 98 connected in parallel with the primary winding of the main transformer 94. By this arrangement, the clamping transistor 98 will be appropriately turned on and turned off according to the operating mode of the pulse width modulator so as to dynamically absorb the flyback magnetic current. However, this kind of switching power supply also generates a serious saturation current when the load varies, as shown in FIG. 7D. Further, great noise will occur due to the saturating peak current applied on the clamping transistor 98. Particularly, in the case of a low load, the switching power supply has the lowest efficiency due to the switching loss in the clamping transistor.
The present invention provides an improved constant-voltage clamping forward conversion switching power supply to mitigate and/or obviate the aforementioned problems.